Since the first electrophotographic machines were developed in 1938, they have found widespread use in document processing. Most of the copiers and printers nowadays used in offices are based on this technology. Owing to its great commercial value, considerable research effort has been devoted to electrophotography and relevant materials.
The key component in an electrophotographic device is the photoreceptor, on which the electrostatic latent images will be generated, which are then transferred onto paper. The entire electrophotographic process comprises the steps of charging of the photoreceptor, imagewise discharge of the photoreceptor, development by toner, transferring the toner image to a sheet of paper, and fixing the toner on the paper by fusing (see Paul M. Borsenberger; David S. Weiss Organic Photorecptors for Xerography; Marcel Dekker, Inc., 1998, Chapter 1).
The photoreceptor usually consists of a charge generation layer (CGL), in which free charge carriers are generated upon illumination, and a charge transport layer (CTL), in which the free charge carriers are transported to discharge at the surface. The CTL essentially determines the discharge speed and thus the printing speed of the device, the mechanical robustness and the chemical stability.
In electrophotographic devices of the negative charge type, hole transport materials (HTM) are used as charge transport material (CTM) in the CTL. A typical, widely used organic CTL comprises a mixture of a binding polymer and a CTM, wherein the binding polymer provides the mechanical robustness and the CTM provides the charge transport function. For example, organic systems like polycarbonate (PC) doped with N,N′-Diphenyl-N,N′-bis-(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) have been successfully used in the CTL of such devices. However, in these devices it was observed that the negatively charged species on the surface can be harmful for the stability of the CTL, which reduces the lifetime of the device.
Electrophotographic devices of the positive charge type, on the other hand, have several advantages over the negative charge type, like for example better resolution and better lifetime of the CTL. These devices require the use of an electron transport material (ETM). However, presently the commercially used ETMs are all inorganic materials, like for example selenium based materials, which are expensive and cannot be used in a flexible device due their brittleness, thus limiting their use in high performance printing system.
It is therefore an aim of the present invention to find alternative and improved materials for electrophotographic devices, in particular improved ETMs for electrophotographic devices of the positive charge type, which have high electron mobility and low dark decay, and are suitable for a high performance printing system. Another aim is to extend the pool of ETMs for use in electrophotographic devices available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing organic materials and electrophotographic devices as described hereinafter.